Posted: Jul 24, 2017 4:05 pm
by Calilasseia
Actually, the ribosome is itself the product of gene expression, viz:

The rRNA is transcribed, at a high speed, in the nucleolus, which contains all 45S rRNA genes. The only exception is the 5S rRNA which is transcribed outside the nucleolus.

The above applies to eukaryotes. A related (but in detail, substantively different) mechanism performs the task in prokaryotes.

In the case of eukaryotes, we have this exposition:

In contrast, eukaryotes generally have many copies of the rRNA genes organized in tandem repeats; in humans approximately 300–400 repeats are present in five clusters (on chromosomes 13, 14, 15, 21 and 22). Because of their special structure and transcription behaviour, rRNA gene clusters are commonly called "ribosomal DNA" (note that the term seems to imply that ribosomes contain DNA, which is not the case).

The point being made in Rumraket's post, is that many copies of the genes encoding for ribosome components exist. Because of the ubiquity of ribosome usage, having many copies of the genes in question allows for mass production of ribosomes, that would not be possible if only one copy of the genes in question were present. Moreover, because all of those genes are in use, and are frequently essential, they tend to be subject to purifying selection, hence the high degree of conservation thereof across lineages. Though gene deletion experiments in various model organisms have alighted upon some that are apparently non-essential, the majority are essential, and deletion of those genes is invariably lethal.